Hey, we're actually using Asterisk. Out of curiousity, what are you using your fax server for?
I have a small 50 line ruby script that converts attachments from emails into outgoing faxes, and a 10 line ruby script that converts incoming faxes into emails with a pdf attachment.
You should try freeswitch instead -- the rxfax thing from Asterisk would sometimes truncate incoming faxes. Not sure why.